The present invention relates to a beam expander for providing coupling between a semiconductor optical device and associated optical fiber and, more particularly, to a double layer beam expander integrated with the semiconductor optical device.
Within recent years, the number of electronic applications that employ optical devices has been rapidly increasing. Typically, optical fibers are used to carry light signals from (or to) a variety of optical devices, such as lasers, amplifiers, modulators, splitters, multiplexers/demultiplexers, routers, switches and photodetectors. As is well known, the use of optical fibers and devices leads to higher data throughputs and increased communication channel bandwidths. These higher data rates result in an ever-increasing concern for maximizing the coupling efficiency between the optical device and associated fiber. In particular, low fiber coupling efficiency of laser diodes has been a major limitation for high power single mode fiber output. In a conventional laser diode, optical confinement in the semiconductor structure is asymmetric and the propagating mode profile is elliptical in shape. Also, the mode profile of these high power diode laser sources results in large beam divergence. The stronger divergence is normally in the vertical (i.e., transverse) direction due to the strong optical confinement in the vertical direction in the layered semiconductor laser structure, as opposed to the weaker optical confinement in the horizontal (i.e., lateral) direction.
This highly divergent, elliptical laser diode output beam profile presents a difficulty when attempting to couple the light from a high power laser diode source to a single mode optical fiber. This difficulty is primarily due to the large mode mismatch between the highly divergent semiconductor laser source and the small numerical aperture optical fiber. For example, the laser spot size is typically around 1 xcexcm, while that of a fiber core is around 10 xcexcm. This disparity limits the coupling efficiency between these two devices to about 10%, if perfectly aligned. Thus, a high power laser diode with a circular mode profile and a narrow far-field divergence is particularly desirable for efficient fiber coupling.
A number of techniques have been developed to increase the coupling efficiency between a semiconductor laser and an optical fiber. These include modifying the shape of the fiber end (e.g., tapering, lensing) so that the modal mismatch is reduced. However, the coupling efficiency is improved at the expense of very tight alignment tolerances. Untapered fibers have also shown to perform well, but there is a need for an additional lens between the laser and a modified end face fiber, increasing the difficulty of assembly. Still another approach is to modify the laser structure so that it has a tapered output section, thus increasing the spot size in the junction plane. As a variation of this approach, a tapered waveguide may be positioned between the laser and the fiber. The tapered waveguide comprises a layer with a low refractive index and is monolithically integrated with the laser. The composition of this layer is critical, since if the effective index mismatch between the laser and the tapered waveguide is high, a significant fraction of light is reflected or lost due to scattering. Alternatively, if the refractive index of the tapered waveguide is matched with the refractive index of the laser, lateral and/or transverse variation of its thickness is necessary to achieve mode expansion. Moreover, to obtain high coupling efficiency at the waveguide joint and large mode size at the tapered waveguide output, a variation of thickness in the tapered waveguide by a factor of approximately four is required.
Thus, a need remains in the art for an arrangement for improving the coupling efficiency between a semiconductor optical device and a fiber that is relatively easy to manufacture and include in an assembly, yet provides the desired high coupling efficiency.
The need remaining in the prior art is addressed by the present invention, which relates to a beam expander for providing coupling between a semiconductor optical device and associated optical fiber and, more particularly, to a double layer beam expander integrated with the semiconductor optical device.
In accordance with the present invention, a semiconductor optical device structure is formed to include a double layer beam expander disposed at the output thereof. The beam expander comprises a first layer of high index material (e.g., InGaAsP) to provide high coupling efficiency and a second layer of low index material (e.g., In1-xGaxAsyP1-y) to provide the required large mode size. In particular, the refractive index of the second layer will depend upon the values of x and y, 0 less than x less than 1 and 0 less than y less than 1, since in the extreme case InP cannot be used as a waveguiding layer. The first layer is deposited to taper away from the laser endface facet and the second layer is formed to cover the first layer. In an exemplary embodiment of the present invention for use with a laser having an emitting wavelength of 1.55 xcexcm, the first (high birefringence) layer of the double layer beam expander may comprise InGaAsP with a refractive index of 3.34, and the second (low birefringence) layer of the double layer beam expander may comprise In1-xGaxAsyP1-y with a refractive index of 3.28.
In one exemplary method of forming the double layer beam expander of the present invention, a first mask is used to cover the optical device region and a second mask is used to define the terminating endface of the first layer, with the first layer then deposited in the region between the first and second masks. The second layer is deposited after the second mask is removed. In an alternative method of forming the double layer beam expander, a first mask is used to cover the optical device and the first layer is blanket deposited on the remaining substrate surface. A second mask is then used to cover the predefined extent of the first layer and an etch step is used to remove the exposed layer. The second mask is then removed and the second layer is deposited.
Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.